Diffusion charging refers to a particle charging process wherein airborne particles are electrically charged in a particle charging section by collisions with gaseous ions (typically positive ions) that have been generated by an ionization electrode such as a needle-tip electrode or corona wire.
A known particle sensor of this type typically comprises a means for establishing an airflow through the sensor (for example a ventilator, a fan or a pump). The airflow passes through the particle charging section, and then through a particle precipitation section for removing substantially all airborne particles from the flow.
The sensor further comprises a particle measurement section having a current meter for measuring the electrical current that results from the deposition of particle-bound charge per unit time in the particle precipitation section.
From the measured electrical current a so-called apparent ultrafine particle number concentration can be calculated. The apparent ultrafine particle number concentration is equal to the ratio of the particle length concentration (i.e. the total length of the string of all airborne UFPs in a unit air volume when they would be lined up therein as a string) and a predetermined average particle diameter.
The particle precipitation section may comprise a mechanical particle filter disposed within a Faraday cage, or a parallel-plate electrostatic particle precipitator. In the case of a parallel-plate electrostatic particle precipitator, charged positive particles are precipitated at the negative plate of the capacitor and the resulting current measured is proportional to the particle concentration times particle diameter.
The particle charging section of the ultrafine particle sensor is for example designed with a high voltage ionization electrode in the form of a needle-tip electrode that is surrounded by an electrically conductive enclosure. The enclosure is at least partly provided with openings. By applying a potential difference between the needle-tip electrode and the electrically conductive enclosure, gaseous ions can be generated that are drawn from the needle-tip electrode towards the enclosure, to escape through the openings establishing a region containing gaseous ions next to the enclosure. The enclosure forms a corona ionization chamber and the gaseous ions emanate through a metal grid into the adjacent gas flow channel.
When an air flow carrying the ultrafine particles passes through this region the airborne particles are electrically charged by diffusion charging.
It turns out that for such a construction, the performance of the needle-tip electrode reduces over time. It has been found out that this is due to a contamination of the electrode's outer surface by compounds that are present in the surroundings of the electrode. For example, silane compounds may form a silicon dioxide layer on the outer surface of the ionization electrode which acts as an electrically insulating layer. Such contaminants are typically present in the airflow carrying the ultrafine particles that are to be monitored.